Was that stated however? I can't find any evidence personally but I could be misremembering too.
They were shown to be fireproof when the Vinsmokes used them at the tea party. There was that fire user who attacked them, Basskarte or something? His flames were useless against their suits.
Apparently Reiju blocked the fire partially but was still damaged by Prometheus and i think it was due to Prometheus being a living flame, a soul infused, mythical flame. Similar properties to Marco's flames, which have their own unique properties that could hurt Prometheus too.
Either the flames were too hot for the raid suit to block or and it was due to their mythical properties.
That's the best explanation I have atm.
To think of it, I don't think Sanji has used Diable Jambe with the suit equipped, which is weird but probably being saved for vs Black Maria atm.
:zorothink: